Mineral hydroxides are an important class of polymer fillers used in particular as flame retardants. Aluminium trihydroxide and magnesium dihydroxide are the major examples of this class of fillers. However, these hydroxides as well as other finely powdered polymer fillers or additives present problems in handling and particularly in compounding into polymers. Ideally, a finer particle size of solid polymer additives should lead to better dispersion in the polymer matrix, and better dispersion equates to more efficient, uniform performance and improved polymer physical properties. Therefore, the solid polymer additives often have particle sizes reduced to less than 10 μm. On the other hand, finer particles are often more difficult to disperse and problems of reagglomeration occur;
Additionally the general handling of finely powdered polymer additives presents particular problems. One substantial problem of finely powdered polymer additives is dusting. The creation of dust involves loss of raw material, increased clean-up costs, and health concerns for those handling the solids.
Another problem is bulk density. Finer solids tend to have decreased bulk density and increased packaging size, volume and cost. The fluffy nature and low bulk density of finely powdered polymer additive solids adversely affects additive flow properties, making them more difficult to meter when using continuous compounding equipment, such as twin screw extruders, but also making general handling difficult. More specifically, poor solids mixing homogeneity results in poor performance in general, for example poor physical properties in the final product. Finer solids tend to lead to, for example, poor physical properties in the final product.
One prior art approach to increasing the mixing homogeneity in the addition of low bulk density solids to polymers involves adding a liquid, such as a plasticizer, to the powder, prior to mixing the powder with the polymer.
Blending the additive powder into the polymer in the form of a masterbatch concentrate that can be diluted with more polymer to achieve the desired final concentration of powder additive is a further common approach. It decreases dusting during the ultimate polymer processing step. However, it not only adds a costly additional step, but it also does not deal with the problem of poor mixing of a low bulk density additive powder and a polymer in forming the masterbatch concentrate. In fact, the masterbatch sometimes has poorer homogeneity because a higher proportion of incompatible fine powder is added. This method also has a disadvantage for fillers which are used in substantially larger amounts, such as flame retardants, due to the large amount of polymer carrier that is included in the final compound.
The approach described in U.S. Pat. No. 4,849,134 to solving these problems is cold compaction of the filler. The disadvantage of this method is that compaction (re-)agglomerates the fine particles of the additive. Unless subsequent polymer processing conditions result in complete breakup of the coarse compacted, i.e. agglomerated particles and dispersion into the polymer, any advantage of the fine particles is lost.
The aim of increasing the bulk density of fine polymer fillers and in particular flame retardants, flame retardant synergists, blends thereof, and other powdered polymer additives has significant value. These additives are included in an amount of about 1% by weight to about 60% by weight, often 10-40% by weight, into a finished polymeric article.
Certain advantages of a lower bulk density polymer additive upon processing of one polymer, PVC, are referred to in U.S. Pat. No. 3,567,669. This patent discloses a high speed mixing process which requires a temperature of at least 170° F. Under these conditions, the PVC particles have a slightly sintered or glazed surface. Solid additives are absorbed or adsorbed onto the polymer surface.
U.S. Pat. No. 3,663,674 discloses densification of poly-α-olefins. Such poly-α-olefins are prepared in a dense granular form suitable for moulding or extrusion by the application of sufficient mechanical energy to compress and collapse the porous polymer particles recovered from the polymerisation reactor. Cited advantages of increased bulk density are improved handling characteristics and the lack of a thermal history prior to processing. No mention is made of the effect of the bulk density of powdered additives upon the processability or properties of the polymer. Nor is there any mention of the use of flame retardants or flame retardant synergists.
Based upon the teachings of the U.S. Pats. No. 3,567,669 and 3,663,674 it was therefore the object of the present invention to provide new concentrates of polymer additives, in particular fillers and flame-retardants, and polymers which new concentrates possess an enhanced processability, show an improved dispersability of the additive throughout the polymer and eventually result in improved properties of the formed parts prepared by processing polymer compounds containing the new concentrates.